More robust common_config_file_iterator

common_config_file_iterator::common_config_file_iterator has a little problem: One of the args is std::set, which is incompatible between libc++ and libstdc++. As a result, the function, if compiled with {GCC, Clang}, is incompatible with {Clang, GCC). This can be fixed without breaking anything by moving it to the header part and declaring it static. What are your opinions about it?